A term that broadly refers to all judges or a judicial officer that assists U.S. district court judges in preparing cases for trial.
Merriam-Webster Online Dictionary
magistrate (noun)  an official entrusted with administration of the laws  as
a)   a principal official exercising governmental powers over a major political unit (as a nation)
b)   a local official exercising administrative and often judicial functions
c)   a local judiciary official having limited original jurisdiction especially in criminal cases 
Merriam-Webster Online Thesaurus
magistrate (noun)a public official having authority to decide questions of law
SYNONYMS:
adjudicator, arbiter, arbitrator, referee, umpireRELATED WORDS:
jurist, justice, magistrate; intermediary, intermediate, mediator, mediatrix, moderator, negotiator; conciliator, go-between, peacemaker, reconciler, troubleshooter; decider